The present invention relates to control equipment for a centrifugal separator for separating a light liquid having a relatively low density and a heavy liquid having a relatively high density from a mixture containing these two liquids. The liquids may, for instance, be constituted by oil and water. The control equipment is intended for a centrifugal separator comprising a rotor, which is rotatable around a rotational axis and forms an inlet for said mixture and a separating chamber, which communicates with the inlet and which has a radially inner part and a radially outer part, said parts being adapted during a separating operation to contain separated light liquid and separated heavy liquid, respectively.
A centrifugal separator of this kind may have outlets for the separated liquids formed in several different ways. Thus, the rotor may be provided with so-called overflow outlets for both of the liquids or an overflow outlet for one liquid and another kind of outlet for the other liquid. An outlet of such another kind may be constituted, for instance, by a non-rotatable so-called paring member or by nozzles situated in the surrounding wall of the rotor. Nozzles are used as a rule when the supplied mixture in addition to said two liquids also contains solids which are heavier than the two liquids. Then, separated solids together with part of the heavy liquid may be discharged through nozzles placed at the periphery of the rotor, whereas the separated light liquid is discharged from a central part of the rotor through an overflow outlet or a paring member. In these cases the rotor can also form a space, which communicates with the radially outer part of the separation chamber in a way such that during a separating operation it will contain separated heavy liquid but not separated light liquid. An excess of separated heavy liquid, which does not leave the separation chamber through said nozzles, is then discharged from the rotor through this space.
Another type of centrifugal separator, in which solids as well as two different liquids may be separated, is a so-called decanter centrifuge. In a centrifugal separator of this kind there is arranged within the rotor a so-called sludge conveyor, which is adapted to transport to a sludge outlet separated solids along the surrounding wall of the rotor. The sludge outlet is often situated at a level in the rotor radially inside the level of the outlets for the two separated liquids.
In a nozzle centrifuge of the above described kind as well as in a decanter centrifuge having a sludge conveyor it may be difficult during a separating operation always to maintain an interface layer, which is formed in the rotor between the liquids separated therein, at a predetermined radial level. The reason for this is that an uncontrollable amount of separated heavy liquid per unit of time leaves together with the separated solids through the so-called sludge outlet of the rotor. If this uncontrollable amount of heavy liquid would exceed the amount of heavy liquid, which per unit of time is introduced into the rotor together with the mixture to be treated therein, the interface layer in the separating chamber between light liquid and heavy liquid will move radially outwardly, and finally separated light liquid will be lost together with the separated solids, when these leave the rotor through the sludge outlet.
A particular separating operation, in which this has caused a problem, is cleaning of oil from sand and water in connection with recovery of oil from so-called oil sands. In this connection nozzle centrifuges are used in at least two separating steps.
In a first separating step a mixture of oil, water, solvent and sand residues is introduced into a nozzle centrifuge, and in addition to the mixture a large amount of water is supplied to the centrifuge. The sand and the main part of the supplied water leave the centrifuge rotor through its nozzles, whereas part of the water is removed from the rotor through a central overflow outlet. Separated oil and solvent are conducted out of the rotor from a central part thereof through a paring member and are pumped further to another nozzle centrifuge to go through a second separating step. Said water being added separately in the first separation step is added in excess, so that the interface layer formed in the separating chamber of the rotor between oil and water shall not be displaced radially outwardly, even after many hours"" operation of the centrifugal separator, when its nozzles have become worn of the outflowing sand and, therefore, let out more water per unit of time than at the beginning of the separating operation.
After the first separating step the oil contains in addition to solvent still residues of sand and water. For obtainment of a separating result as good as possible there has been developed for controlling the separating operation in the second separating step a particular control equipment. By means of this control equipment it is possible to avoid continuous addition of an excess amount of water to the mixture being introduced into the centrifugal rotor. Instead, there is introduced into the separating chamber of the rotorxe2x80x94only when this is needed and only in a required amountxe2x80x94water through a space in the rotor of the kind as previously described, i.e. a space communicating only with the radially outer part of the separating chamber. Through the same space water is also removed from the rotor during periods when an excess of water enters together with the oil to be cleaned, which excess of water cannot leave the rotor through the sludge outlet nozzles.
Said control equipment, which has been developed particularly for the second separating step, is expensive and complicated, however. Thus, it comprises for each one of a great number of nozzle centrifuges a pressure vessel for water. The lower part of the pressure vessel communicates through a conduit with a liquid transferring member, which is situated in said space in the rotor of the centrifugal separator, for the introduction of water into or discharge of water out of the rotor. In the upper part of the pressure vessel there is maintained a gas pressure (usually by means of nitrogen gas), the magnitude of which is continuously controlled in response to the amount of water which at each moment is present in the pressure vessel, so that the liquid pressure at the bottom of the pressure vessel and thus within the conduit, through which the pressure vessel communicates with said space in the centrifugal rotor, is always kept constant at a predetermined value.
The constant value of the liquid pressure in said conduit corresponds to a desired radial level in the separating chamber of the rotor for the interface layer formed therein between separated oil and separated water. If the interface layer moves radially outwardly from the desired level, the pressure drops in said space in the rotor, the result of which is that water is pressed from the pressure vessel through said conduit into the rotor, until the interface layer has returned to the desired radial level. A level-sensing member in the pressure vessel is adapted to initiate upon need the supply of new water to the pressure vessel, so that it will never be empty of water.
If the interface layer in the separating chamber of the rotor starts to move radially inwardly from the desired level, the pressure in said space in the rotor increases, excess of water being pressed from this space through said conduit into the pressure vessel. When the liquid level in the pressure vessel has risen to an upper limit level, a bottom outlet of the pressure vessel is opened for release of water therefrom.
The object of the present invention is to provide a simple and inexpensive control equipment for a centrifugal separator of the initially described kind, in the rotor of which a space of the above discussed kind is delimited.
This object can be obtained by means of a control equipment including
a supply device for supply to the rotor of a control liquid having a density higher than that of said light liquid, said supply device having a pressure source for supplying pressurized control liquid and a supply conduit, which at its one end is connected to the pressure source for receiving pressurized control liquid and at its other end is connected to a liquid transferring member for introducing pressurized control liquid into the rotor, the supply device further being adapted upon need to supply control liquid to the rotor only in an amount per unit of time such that is required for avoiding that an interface layer formed in the separating chamber between separated light liquid on one side and separated heavy liquid or control liquid on the other side moves radially outwardly from a predetermined radial supply level, and
a discharge device for discharge of separated heavy liquid and/or control liquid from said space in the rotor, the discharge device having a discharge conduit and being adapted, when the rotor is charged with an excess of heavy liquid, to discharge separated heavy liquid and/or control liquid from the rotor through said discharge conduit in an amount per unit of time such that is required for avoiding that said interface layer moves radially inwardly from a predetermined radial discharge level.
According to the invention a control equipment of this kind is characterized in that the discharge device is arranged to discharge liquid from said space in the rotor a different way than through said supply device.
The control equipment according to the invention distinguishes from the previously described known control equipment principally in that the pressure source for control liquid, which is part of the supply device, is not integrated in the discharge device. The separated heavy liquid and/or control liquid leaving the rotor, thereby, need not be accumulated at an elevated pressure and consequently no pressure vessel is needed. Also, there is no need for a system for compression of gas and for control of the pressure of such a gas. Instead, the pressure source may be constituted by a simple liquid pump and the whole control of the supply of controlling liquid and discharge of separated heavy liquid and/or control liquid can be performed by means of a so-called constant pressure valve, preferably, however, two constant pressure valves. If a container is needed for a buffer amount of control liquid, such a container may be free of pressure and common to several centrifugal separators. If desired, control liquid may be reused in that at least part of the liquid leaving the rotor through said discharge conduit is conducted to a common container of this kind.
Said control liquid may be of the same kind as the separated heavy liquid, i.e. usually water. Further, depending upon which components are included in the control equipment, the predetermined radial supply level for the interface layer in the separating chamber between separated light liquid and separated heavy liquid may be the same as or somewhat differing from the predetermined radial discharge level for this interface layer. Preferably, a certain radial movement of the interface layer is admitted, since a more stable control of the supply and discharge of liquid is thereby facilitated.
The supply of control liquid to the rotor may be made to any suitable part of the rotor. However, in a preferred embodiment of the invention the previously mentioned space in the rotor is used both for the supply of control liquid to the rotor and for discharge of separated heavy liquid from the rotor. Separate members may be arranged for the supply of liquid to and the discharge of liquid from this space, but preferably said liquid transferring member for introducing control liquid into the rotor may be used also for discharge of liquid from the rotor, the liquid transferring member preferably forming a channel, through which said supply conduit as well as said discharge conduit communicate with said space in the rotor. The liquid transferring member then may include a so-called paring member or, for instance, include at least two stationary circular discs, which are arranged coaxially with the rotor and axially spaced from each other in said space. Liquid may be supplied and discharged through a central opening in one of the discs, the space between the discs communicating with said space in the rotor at the periphery of the discs. A liquid transferring member of this kind, used merely for discharge of a liquid from a centrifugal rotor, is described in SE 76 670 (from the year 1930).
A liquid transferring member of this kind may be used in a rotor of a so-called open type, i.e. a rotor in which a free liquid surface is maintained in said space. However, the invention can be used also in a so-called hermetically closed rotor, i.e. a rotor in which a space of said kind is kept completely filled with liquid during the operation of the rotor and said liquid transferring member is constituted merely by a central part of the rotor or by a stationary member adapted to seal against a central part of the rotor.
In a particular embodiment of the invention said discharge device in connection with a rotor of the so-called open type may include a discharge member, which is arranged radially movable in said space in the rotor, so that the position of a free liquid surface in said space may be chosen and may be adjusted according to need, e.g. with regard to the relevant density of the separated liquids. Thus, the radially movable discharge member may be constituted for instance by a paring member of the kind known from WO 97/27946. By means of a discharge member of this kind a varying excess of separated heavy liquid in the rotor may be discharged and the liquid surface in said space in the rotor may be prevented from moving radially inwardly from a predetermined radial level.
If a similar or the same liquid transferring member is used for supply of control liquid to said space, the liquid transferring member can be allowed to move radially during a separating operation and to follow possible movements of the liquid surface therein radially outside said predetermined level. Then, the supply device for supply of control liquid to the rotor may be formed such that control liquid is supplied to the rotor as soon as the liquid transferring member tends to move radially outwardly from the predetermined level. Possibly, the supply of control liquid to the rotor may take place through a supply member separate from a radially movable liquid discharge member. If so, the latter could be used as a floater, which is coupled in one way or another to the supply device and adapted, in response to its radial movement or its radial position, to control the supply of control liquid in a way such that the free liquid surface is maintained at the predetermined radial level. As mentioned, however, one and the same liquid transferring member is preferably used for both supply and discharge of liquid to and from, respectively, the rotor.
For avoiding that the liquid surface in said space in the rotor moves radially inside the predetermined level, the rotor may have an overflow outlet in said space. Liquid flowing over this overflow outlet may either be allowed to leave the rotor directly or be caught in an outlet part of the space and be conducted out of the rotor through a non-rotating discharge member, e.g. a paring disc.
In case an overflow outlet of the kind just mentioned is not used but the liquid is conducted out of said space in the rotor directly through a non-rotating discharge member, the previously mentioned discharge conduit with which the discharge member is connected preferably contains an outlet valve, which is controllable in a way such that it maintains a desired predetermined liquid pressure in the discharge conduit upstream of the outlet valve. Valves of this kind, which are previously well known under the name constant pressure valves, are adapted to let through a liquid flow of a varying magnitude while maintaining a constant pressure upstream of the valve. A valve of this kind gives the same result in said space in the rotor as an overflow outlet arranged therein for liquid flowing out from the rotor separating chamber, i.e. it prevents a free liquid surface in the space in the rotor from moving radially inside a certain predetermined radial level.
Correspondingly, said supply device for the supply of control liquid may be provided with means which automatically supply control liquid to the rotor only in an amount per unit of time such that the free liquid surface in the space in the rotor does not move radially outwardly from the predetermined radial level therein. Even in this case a so-called constant pressure valve may be used, which is then situated in said supply conduit and adapted, independently of the magnitude of a liquid flow admitted therethrough, to keep the liquid pressure downstream of the valve at a desired predetermined value. A precondition for this is that the supplied control liquid in the supply conduit downstream of the valve has hydraulic contact through the previously mentioned liquid transferring member with the liquid rotating with the rotor in said space therein. If so, namely, the value of the liquid pressure in the supply conduit constitutes a measurement of the radial level of the free liquid surface in this space. A relatively high liquid pressure in the supply conduit, thus, corresponds to a relatively small radial distance between the free liquid surface and the rotational axis of the rotor, whereas a relatively low liquid pressure in the supply conduit corresponds to a relatively large distance of this kind. If the liquid pressure in the supply conduit would exceed a desired or a predetermined value, the valve closes completely for through flow.
Even in connection with a so-called hermetically closed rotor constant pressure valves of the above described kind may be used. Even in a case like this the magnitude of the liquid pressure in the supply conduit and in the discharge conduit becomes a measurement of the radial level of the interface having been formed in the separating chamber of the rotor between separated heavy liquid and separated light liquid.
In a preferred embodiment of the invention a liquid transferring member in the one flow direction communicates with said space in the rotor and in the other flow direction communicates with said supply conduit as well as said discharge conduit. In the supply conduit there is situated an inlet valve in the form of a first constant pressure valve, adapted to let through a variable amount of pressurized control liquid from the previously mentioned pressure source to the liquid transferring member only in an amount per unit of time such that the liquid pressure in the supply conduit downstream of the inlet valve does not drop below a predetermined first value. Further, there is placed in the discharge conduit an outlet valve in the form of a second constant pressure valve, which is adapted to let through a variable amount of liquid in a direction away from the rotor only in an amount per unit of time such that the liquid pressure in the discharge conduit upstream of the outlet valve does not rise above a predetermined second value. The predetermined first value may coincide with the predetermined second value, but preferably a certain difference exists between the values, whereby a better co-operation is obtained between the control function performed by the inlet valve and the control function performed by the outlet valve.
If the predetermined first value, i.e. the pressure value for the opening of the inlet valve, is somewhat lower than the predetermined second value, i.e. the pressure value for the opening of the outlet valve, the free liquid surface in said space in the rotor is allowed to move within certain limits without any liquid flow at all coming up through said liquid transferring member. If, instead, the pressure value for the opening of the inlet valve is somewhat higher than the pressure value for the opening of the outlet valve, a certain flow of liquid will always take place from the supply conduit to the discharge conduit.
If a pressure source can be provided, which delivers control liquid having exactly a desired pressure independent of the magnitude of a supplied flow of control liquid, it would be required in the control equipment according to the invention only one single constant pressure valve, i.e. the one in the discharge conduit. If so, this would be able to perform the function to prevent a liquid flow in the undesired direction, i.e. from the rotor back to said pressure source through the supply conduit.
In addition to the control equipment described above the invention also relates to the general method, in connection with a centrifugal separator of the initially described kind, of removing liquid from said space in the rotor a different way than through said supply device, when the rotor is charged with an excess amount of heavy liquid.